1. Simulating the 802.11a PHY Model: Making It Accurate Jeongkeun Lee, Sung-Ju Lee, HP Labs Jiho Ryu, Taekyoung Kwon, Seoul National University Physical Layer Capture 802.11 Frame Receive Procedure If SINR(2nd) > CAPTURE_Thres Second Frame Capture (SFC) : drops the 1st frame’s preamble and tries to detect the 2nd frame’s preamble Otherwise, First Frame Capture (FFC) : continue to detect the 1st frame’s preamble Un-captured frame signal increases the interference power Net TCP Throughput Performance Carrier Sense Simulator Model Revision Preamble Detection (PD) PHY holds the CS busy for the duration indicated in the PHY header Energy Detection (ED) If the preamble portion was missed, the PHY holds the CS busy for any signal above CSThresh 1)Detects and measure signal energy 2)Timing synchronization Frame format: Receiver actions: 1)Read bitrate, frame length info etc. 2)Parity bit check 3)Go into Receiving state (Lock on) 1)Read payload 1)Frame error check Three steps for a successful frame reception 1) Preamble detection 2) PHY header reception 3) MAC CRC check Preamble Capture Receiver has already locked on the 1st frame (in Receiving state) If Message-In-Message (MIM) mode is implemented and SINR(2nd) > CAPTURE_Thres Unlock the 1st frame and begin to re-synchronize with the 2nd frame (SFC) W/O MIM mode, MAC frame capture is not possible. MIM mode implementation is chipset-dependent Atheros chipset supports MIM mode Prism chipset does not MAC Frame Capture 11b11g11a CS modePD, ED, or PD+ED PD+ED CSThresh= RX_sensitivity = RX_sensitivity + 20dB (standard) = RX_sensitivity + 10dB (in Atheros) PHY3: PHY2 + MAC Frame Capture PHY1: PHY0 + SINR-based preamble detection PHY2: PHY1 + Preamble Capture (CSThres=RX_sensetivity) CSThres = RX_sense + 20dB Ex) A NEW frame arrives, RX_sens < rss(NEW) < RX_sens+20dB, sinr(NEW) < CAPTURE_Thres - In real systems with preamble detection and high CSThres (PHY1), PD and ED fail -> Stay in idle state - In original Qualnet model (PHY0), the receiver goes into the receiving state but it will fail the CRC check Conclusion PHY0: Original Qualnet 3.9.5 The aggregated TCP throughput can increase up to 430% with the revised PHY model Extended version will be presented at ACM MSWiM 2008 Motivation Measurement-based testbed studies show operations of frame reception, capture effect and carrier sensing in real IEEE 802.11 systems differ from those of popular research simulators Different chipset vendors implement physical layer capture differently Revise the 802.11a PHY model simulation model SINR-based Preamble Detection Second Frame Capture : preamble capture + MAC frame capture Carrier Sense threshold Observe the effects of the revised models on network performance Selected References [Wintech07Lee] J. Lee, W. Kim, S. J. Lee, D. Jo, J. Ryu, T. Kwon, and Y. Choi. “An Experimental Study on the Capture Effect in 802.11a Networks,” ACM WiNTECH, Montreal, Canada, Sept. 2007. [ICNP04Kochut] A. Kochut, A. Vasan, A. Shankar, and A. Agrawala. “Sniffing out the correct physical layer capture model in 802.11b,” IEEE ICNP, Berlin, Germany, Oct. 2004. 1 st 2 nd 1 st 2 nd PHY1: 350% increase over PHY0